FIG. 9 is a highly schematic plan view of a signal switching device 100 incorporating two field effect transistors (FETs) 17 and 18. The signal switching device of FIG. 9 includes an input line 2, first and second output lines 3 and 4, and control lines 5 and 6. For clarity, the input line 2 is labelled I and the first and second output lines 3 and 4 are labelled 01 and 02, respectively. The input, output, and control lines are electrically conducting metallizations or stripes, such as striplines, disposed on the substrate for conducting direct current and relatively high frequency signals. The input, output, and control lines as well as the FETs 17 and 18 are disposed on a semiconductor substrate 20 that is schematically illustrated in FIG. 9. Output lines 3 and 4 are disposed at opposed ends of the signal switching device and are perpendicular to the input line 2. In response to a control signal applied to control line 5, FET 17 controls whether an input signal supplied to input line 2 reaches the output of line 3. Likewise, the FET 18 controls, in response to a control signal applied to the control line 6, whether the input signal is transmitted to the output line 4.
The structure of the FET 17 is illustrated in detail in the plan view of FIG. 10. The FET 18 is symmetrical to the FET 17 and does not need separate illustration for understanding. In FIG. 10 and in the other figures, the same elements are given the same reference numbers. An active region 16 is disposed within substrate 20. As well known in the semiconductor arts, if the substrate 20 is a compound semiconductor, for example gallium arsenide, the substrate may be semi-insulating and the relatively low resistivity active region 16 may be formed by the ion implantation or diffusion of dopant impurities into the substrate or by another technique. An input electrode 7 includes a source electrode 7a disposed within the active region 16 on the substrate 20 and a source pad 7b disposed on the substrate 20 outside the active region 16. The source pad 7b is electrically connected to the source electrode 7a through an air bridge structure 12. As well known in the art, air bridge structures include an electrical conductor spaced from the surface of the substrate on which they are disposed that provides an electrical crossover path, i.e., an insulated separation, between two electrical conductors that cross each other. An electrically insulating material may be part of the air bridge structure and interposed between the insulated two electrical conductors but, preferably, in an air bridge structure the two electrical lines are separated by empty space.
An output electrode 14 includes a drain electrode 14a disposed on the semiconductor substrate 20 within the active region 16 and a drain pad 14b disposed on the substrate 20 outside the active region 16. A control electrode 15 includes a gate electrode 15a disposed on the substrate 20 within the active region 16, between the source and drain electrodes 7a and 14a, a gate pad 15c, and a connecting portion 15b connecting the gate electrode 15a to the gate pad 15c. The connecting portion 15b passes underneath the air bridge structure 12 and is electrically insulated from the conducting portion of the air bridge that provides an electrical connection between the source electrode 7a and the source pad 7b. As shown in FIG. 10, the input line 2 is connected to the source pad 7b, the output line 3 is connected to the drain pad 14b and the control line 5 is connected to the gate pad 15c.
The operation of the signal switching device in FIG. 9 is described with respect to the structure shown in FIG. 10 and a symmetrical structure comprising FET 18, although that symmetrical structure not shown in detail in the figures. When an input signal is applied to input line 2 for transmission to the output line 3, the FET 17 is made conductive, i.e., switched "on" by the application of a voltage, such as 0 volts, to the control electrode 15 of the FET 17 through the control line 5. At the same time, the FET 18 is made non-conductive, i.e., turned "off" by applying a voltage, for example -5 volts, to the control electrode of the FET 18 through the control line 6. The input signal is thereby transmitted to the output line 3 and is not transmitted to the output line 4. Likewise, if the control signals applied to control lines 5 and 6 are interchanged, the FET 17 is turned "off" and the FET 18 is turned "on" so that the input signal applied to input line 2 is transmitted to output line 4.
In the signal switching device described, two FETs are needed to switch the input signal between the two output lines. An active region must be formed for each of the two FETs and a relatively large area of the substrate 20 is required for the two FETs. Therefore it is difficult to reduce the area occupied by the signal switching device. In addition, the geometry of the signal switching device, i.e. the positional relationship of the input, output, and control lines, is limited, constraining the placement of components in the design of an integrated circuit including the device.